Orthogonal frequency division multiplexing (OFDM) has become the modulation of choice for higher data rate wireless communication links for personal area networks (PAN), local area networks (LAN) and metropolitan area networks (MAN) networks. OFDM waveforms have both amplitude and phase information requiring linear amplifiers generally having lower efficiency in the transmitter power amplifier (PA). The significant peak to average power ratios, typically 10 dB to 15 dB, further reduces the average efficiency of such OFDM transmitters. Power control on mobile units may further result in an average transmit power that is typically 30 dB to 50 dB lower than the peak power, and a corresponding reduction in efficiency. In mobile and handheld applications, such lower power efficiency in transmit mode may severely affect reliability, for example due to thermal issues, as well as limiting battery life of the hand-held device. Switching power amplifiers, commonly utilized with pure frequency/phase modulation schemes, are capable of achieving a higher efficiency; however the application of switching power amplifiers to OFDM systems is not straightforward.
Furthermore, conventional radio transmitters comprise analog circuits which are sensitive to process, voltage and/or temperature, typically utilize inductors that occupy a larger die area, and/or that are not compatible with scaled lower voltage complementary metal-oxide semiconductor (CMOS) processes, such as headroom/linearity, gain and/or matching constraints. The increasing speed of the lower-voltage transistor can be exploited to replace lower-speed, higher-resolution analog circuits with higher-speed, lower-resolution circuits.
It will be appreciated that for simplicity and/or clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, if considered appropriate, reference numerals have been repeated among the figures to indicate corresponding and/or analogous elements.